Frequency control



FREQUENCY .CONTROL Filed May 23, 1941 IN'VENTOR ATTbRNEY Patented Feb. 15, 1944 FREQUENCY CONTROL Harold 0. Peterson, Riverhead, N. Y., assignor to Radio of Delaware Corporation of America, a corporation Application May 23, 1941, Serial No. 394,762

19 Claims.

This application concerns a new and improved method of and means for automatic frequency control in a. wave receiver of mark/space wave energy transmission, and makes use of dual heterodynes in the discriminator detectors.

Frequency shift signalling wherein wave energy is shifted from one frequency, which may be referred to as the spacing wave, to a second frequency, which may be referred to as the marking wave, is used for the transmission of telegraph signals, including multiplex, and for teletype printers and for photoradio services using CFVD and for other transmissions of this type. The wave energy may be considered as acarrier the frequency of which is shifted from one value to another in accordancewith signals and as two carriers alternately keyed in intensity between a usable value and an unusable value. In the reception of wave energy which is keyed back and forth between a mark and a space frequency there is required a rather specialized form of automatic frequency control because the transmitter frequency may remain for long or short periods of time on either the mark or the space frequency. Automatic frequency control systems known to the radio art are adapted to operate on one wave frequency, whicirmay vary, and would not respond well to wave shift signals of the nature involved here. A purpose of my invention is to provide automatic frequency control means which will operate well on spaced wave signals. In other words my novel system responds equally well to wave length variations in both the spacing and marking frequencies no matter how long they are on.

Figure 1 shows a block diagram of such a receiver and Figure 2 shows a diagram of the frequency relationships in a preferred arrangement of this system.

Referring to Figure 1, the incoming signal is unit 6 from which output energy may be conintermediate frequency circuits of unit 3. The output of intermediate frequency unit 3 is conducted to an amplitude limiter in unit 5. Some ducted to any utilization circuit, such as a recorder.

Some of the output from the limiter in 5 is conducted to filters in units I and 8, which may have characteristics as indicated by curves 25 and 26 of Figure 2. The outputs of the filters in units I and 8 are conducted to heterodyne detectors in units 9 'and II where they are combined with wave energies from oscillators in units I0 and I2 to form wave energies of new frequencies which are transmitted through'a selective circuit in unit 29 to an amplitude limiter in unit I3 followed by a frequency modulation discriminator and detector circuit which operates through automatic frequency control means in unit 23 to adjust the frequency of the oscillations supplied by the circuits in unit 24.

The discriminator circuits may be of the type disclosed in Crosby United States application Serial No. 618,154, filed June 20, 1932, now U. S.

Patent No. 2,229,640, dated January 28, 1941, or,

received on antenna I and conducted to high I frequency apparatus in unit 2 in which the wave energy is amplified and is heterodyned in a mixer tube with oscillation from a source of oscillations in unit 24. The unit 2 includes, if

desired, an intermediate frequency amplifier as shown, may be of the off tuned type. The pri mary winding of a transformer I6 is coupled to two secondary windings, one of which is parallel by a condenser to form a resonate circuit I4, and the other of which is tuned by a condenser to form a resonate circuit I5. Resonate circuit I4 is connected through resistance I9 in series with a rectifier I'I. Resonate circuit I5 is connected through a resistance 2| in series with a rectifier I8. I The resistances I8 and 2| are in series and itis noted that the anode of rectifier I8 is connected to the end of resistance I9 to which the anode of rectifier I1 is connected so that currents through the resistors I9 and 2| are in opposed directions. The resistances I9 and 20 are shunted by by-pass condensers 20 and 22 respectively.

The difference of the potentials set up across I9 and 2| is supplied to means in unit 23 arranged to control the frequency of the oscillations produced in unit 24. This means may comprise a motor, the direction of rotation of which is reversed to rotate a tuning element of the generator circuits in a direction which depends upon the polarity of the difference potential across resistances l9 and 2|.

Preferably the motor is controlled as to rotation and direction of rotation by electron discharge tube means which is in turn controlled by potentials developed across resistances IS and 2|. For example, the motor may have two windings one of which is supplied alternating current somewhat directly and the other of which is fed alternating current by a tube system which includes phase changing means, the phase change being controlled by potentials from resistances l9 and 2| applied to a tube electrode. For more details of this preferred control means see Crosby copending application Serial No. 393,339, filed May 14, 1941. The modulator 23 may comprise a reactance tube operating as disclosed in Crosby United States application Serial No. 136,578, filed April 13, 1937, now U. S. Patent No. 2,27 9,659, dated April 14, 1942. Other forms of automatic frequency control circuits may be used.

In the circuit shown in Figure 1, selective circuit 14 may be tuned to a frequency slightly above the frequency passed by circuit 29 and selective circuit I may be tuned to a frequency slightly below the frequency passed by 29. I1 and I8 represent diodes feeding load resistors l9 and 2| with by-pass condensers 20 and 22. This circuit operates so that when the frequency of the wave energy in 29 drifts in one direction,

less negative or more positive voltage is conducted to control 23 and when it drifts in the opposite direction less positive or more negative voltage is conducted to control 23.

Referring to Figure 2, we may let 21 represent the frequency of the oscillations of the oscillator in II] and 28 represent the frequency of the oscillations of oscillator l2. Curve 25 represents the characteristic of one of the filters I and 8 while 26 represents the characteristic of the other of said filters. It will be noted that the beat between the space frequency S and oscillator frequency 21 is substantially the same as the beat frequency between mark M and frequency 28. Therefore, for either mark or space at the transmitter, the beat frequency fed to selective circuit 29 will be substantially the same. If there is a residue of other frequencies due to the pass band characteristics of filters 25 and 25, not discriminating perfectly between mark currents characteristic of the marking frequency and its side frequencies and currents characteristic of the spacing frequency and its side frequencies, this residue will have less amplitude than that of the desired frequency and will consequently be eliminated in limiter I3 to a considerable extent.

A suitable time constant circuit TC is included in controlunit 23 or in the line L so that the automatic frequency control will not have a tendency to follow at the signalling (keying) frequency. The beat frequency passed by 29 and limited as to amplitude in I3 is fed to the tuned circuits l4 and I5. As long as the frequency of the intermediate frequency energy supplied to filters I and 8 and/or to 29 is correct, a resultant potential is produced (may be zero) across resistances I9 and 2i, which holds the oscillations produced in unit 24 at the selected frequency. If there is a deviation of the mean frequency of the energy at the inputs of 1 and 8 and/or at l3, then a change is caused in the rectifier outputs, resulting in a change in the difference potential across resistances l9 and 2|, which acts through the control means in 23 to change the frequency of the oscillations supplied from 24.

I claim:

1. The method of signaling with energy comprising modulated waves of different frequencies the amplitudes of which waves vary alternately from a usable value to a small unusable value which includes the following steps, separating said waves each from the other, producing current the frequency of which varies in accordance with deviations in the frequency of one of said waves from its normal frequency when said one of said waves is of usable value, producing current of substantially like frequency the frequency of which varies in accordance with the deviations of the other of said waves from its normal frequency when said other of said waves is of usable value, demodulating said produced currents and controlling the frequency of said modulated waves in accordance with the demodulated currents.

2. A method as recited in claim 1 including the step of limiting the amplitudes of said produced currents before they are demodulated.

3. The method of signaling with energy which alternates between a. marking frequency wave and a spacing frequen'cy wave and of stabilizing the frequencies of said waves during said signaling process which includes the following steps, beating said waves with oscillations to produce alternately beat notes of substantially the same frequency one of which beat notes is characteristic of said spacing frequency wave, and the other of which beat notes is characteristic of said marking frequency wave, producing potentials which are characteristic of deviations in frequency of said produced beat notes, and controlling th frequency of said first mentioned waves by said produced potentials.

4. The method of signaling with wave energy comprising a marking frequency wave and a spacing frequency wave and of stabilizing the frequencies of said waves during said signaling process which includes the following steps, separating wave energy of said spacing frequency from wave energy of said marking frequency, producing wave energy characteristic of said wave energy of marking frequency, producing wave energy of substantially like frequency characteristic of said wave energy of spacing frequency, producing potentials which are characteristic of deviations in frequency of said produced wave energies, and controlling the frequency of said first two waves by said produced potentials.

5. The method of signaling with wave energy comprising modulated waves of. different frequencies the amplitudes of which may vary alternately from a usable value to a smaller unusable value and of stabilizing the frequencies of said waves during said signaling process which includes the following steps, separating said waves of different frequencies, heterodyning one of said waves to a lower frequency, heterodyning the other of said waves to substantially the same lower frequency, limiting the amplitudes of said Waves of lower frequencies, producing potentials which are characteristic of deviations in frequency of said limited waves of lower frequency, and controlling the frequency of said wave energy by said produced potentials.

6. In means for producing current the value of which changes in a manner which corresponds to changes in frequency of energy comprising two waves of different frequency and their side bands resulting from modulation of said waves including modulation of one or the other thereof to substantially zero wave amplitude at all times, selective circuits for substantially separating said waves one from the other, means for mixing said waves with other waves to obtain beat notes of substantially the same frequency, and a frequency discriminator circuit including a rectifying syslast mentioned circuits, a frequency discriminating circuit comprising off-tuned circuits and rectifiers with impedance connected with their electrodes coupled to said limiter, and means for controlling the tuning of said tunable circuit in accordance with the potentials supplied by said rectifiers to said impedance.

13. In a system for amplifying and demodulating energy comprising a marking frequency wave means for heterodyning energies of both of said frequencies to a substantially common frequency,

a discriminator circuit including rectifiers coupled with said last named means, and means for controlling the first wave energy frequency in accordance with the potentials at the output of said rectifiers.

9. In a system for receiving wave energy comprising energy of a marking frequency and energy of a spacing frequency separated in the frequency spectrum from said marking frequency,

filter circuits for separating energy of said mark- 7 ing frequency from energy of said spacing frequency, means for heterodyning the separated energies of both of said frequencies to a substantially common frequency, a discriminator circuit including rectifiers coupled with said last named means, and connections for controlling the frequency of the first wave energy in accordance frequency, a wave amplitude limiter coupled to said circuits, a frequency discriminating circuit including rectifiers coupled to said amplitude limiter, and means for controlling the frequency of said first mentioned wave energy in accordance with potentials derived from said rectifiers.

11. In a signaling system for wave energy comprising a marking frequency wave and a spacing frequency wave separated from the marking frequency wave by a band of frequencies including means for stabilizing the frequencies of said waves, selective circuits for separating wave ener'gy of the marking frequency from wave energy of the spacing frequency, two heterodynes for heterodyning the separated wave energies to a substantially common frequency, a wave amplitude limiter coupled to said heterodynes, a frequency discriminating circuit including a rectifier system coupled to said limiter, and connections for controlling the frequency of said wave energy in accordance with the potentials derived from said rectifier system.

12'. In a system for amplifying and demodulating energy comprising a marking frequency wave and a spacing frequency wave, an amplifier including a tunable circuit for amplifying said energy, a filter tuned to the frequency of said marking frequency wave, a filter tuned to the frequency of said spacing frequency wave, said filters each having an input and an output, connections coupling the inputs of said filters to said amplifier, circuits coupled with the output of said filters for heterodyning the wave energies passed thereby to a substantially common frequency, a wave amplitude limiter coupled to said and a spacing frequency wave, means for amplifying said energy, said means including a wave frequency converter and a source of oscillations of controllable frequency, means for amplifying and limiting the amplitude of wave energy, said last named means being coupled to said wave frequency converter, a filter tuned to the frequency of said marking frequency wave, a filter tuned to the frequency of said spacing frequency wave,

said filters each having an input and an output, means coupling the inputs of said filters to said limiting means, means coupled with the output of said filters for heterodyning the wave energies passed thereby to substantially like frequencies, an amplitude limiter coupled with said last named means, a frequency discriminator comprising of!- tuned circuits and rectifiers with impedances connected with their electrodes coupled tosaid last mentioned amplitude limiter, and means for controlling the frequency of said oscillations of controllable frequency in accordance with the potentials supplied from the output of said rectifiers to said impedances.

14. In means for producing current the value of which changes in a manner which corresponds with changes in frequency of wave energy comprising two waves of different frequency and their side bands resulting from modulation of said waves including modulation of one or the other thereof to substantially zero wave amplitude at all times, selective circuits for substantially separating said waves one from the other, two heterodyne systems for heterodyning said separated waves to a common frequency, a rectifier system and a frequency variation responsive circuit coupling said rectifier system to sai heterodyne systems.

15. The method of signalling with wave energy comprising modulated waves of different frequencies the amplitudes of which may vary alternately from a usable value to a smaller unusable value and of stabilizing the frequencies of said waves during said signalling process which includes the steps of separating said waves of different frequency, heterodyning one of said waves to a lower frequency when it is-of usable value, heterodyning the other of said waves to substantially the same lower frequency when it is of usable value, and producing potentials which are characteristic of, deviations in frequency of said waves of lower frequency.

16. In a system for amplifying wave energy including wave energy of a marking frequency and wave energy of a spacing frequency separated in the frequency spectrum from said marking frequency, circuits excited by said wave energies of marking and spacing frequency, for deriving wave energies of like frequency characteristic of said wave energies of marking and spacing frequency, a rectifier system, a frequency responsive circuit coupling said rectifier system to said last named circuits. and connections for controlling the first mentioned wave energy in accordance with the potentials at the output of said rectifier system.

17. In a signalling system for wave energy comprising, wave energy of a marking frequency and wave energy of a spacing frequency separated in the frequency spectrum from said marking frequency, filter circuits for separating wave energy of said marking frequency from wave energy of said spacing frequency, mixing circuits for heterodyning the separated energies of both of said frequencies to characteristic energies of substantially like frequencies, a frequency. discriminator circuit including a rectifier coupled with said mixing circuits, and a control circuit coupled to said rectifiers for controlling the frequency of the first mentioned wave energy in accordance with potentials derived from said rectifiers.

18. In means for producing current the value of which changes in a manner which corresponds with changes in the frequency of wave energy comprising, two waves of different frequency which are alternately modulated by control potentials between a high or usable value and a low or unusable value, selective circuits for substantially separating said waves one from the other, frequency changing systems for deriving from said separated waves characteristic waves of substantially like frequency, a rectifier system, and a frequency variation responsive circuit coupling said rectifier system to said frequency changing systems.

19. In a system for producing currents the intensity of which changes in a manner which cor responds to changes in frequency of wave energy comprising a marking wave and a spacing wave alternately interrupted and spaced in the frequency spectrum by a band of frequencies, selective circuits for separating said waves, a frequency changing system coupled to said selective circuits for producing from saidseparated waves characteristic waves of substantially like frequency, a frequency change responsive circuit coupled to said frequency changing system and a rectifier system coupled to said frequency change responsive circuit.

HAROLD O. PETERSON. 

